It is recent tendency that color images are required even in the field of copying machine or printer. Color image forming methods with high practical value can be roughly classified according to usual called name into a transfer drum method, an intermediate transfer method, a KNC method in which several colored images are piled on a photoreceptor and transferred collectively, and a tandem method.
Of course such the names are each given from different viewpoint, accordingly, for example, a method composed of the intermediate transfer method and the tandem method can be naturally used. The color image forming apparatus by the tandem method is known as an apparatus giving a high quality full color image. In the tandem method, toner images are separately formed on photoreceptors each corresponding to color of yellow, magenta, cyan, or magenta, and the color images are piled on an intermediate transfer member and the piled image is collectively transferred onto an image recording material.
In the tandem image forming method, an image defect caused by imperfect transfer of the toner image tends to occur since the method includes two transfer steps, the first transfer step for transferring the toner image from each of the photoreceptors to the intermediate transfer member and the second step for transferring the image from the intermediate transfer member to the recording paper.
For example, the imperfection of the transfer of the toner from the photoreceptor to the intermediate transfer member tends to cause image defects such as reducing of image density and lacking of transferred image. Besides, it has been reported that the imperfection of the toner transfer from the intermediate transfer member to the image recording paper causes scattering of character image and lowering of sharpness caused by rebound of the toner in the transfer process and periodical defects caused by toner filming on the photoreceptor.
On the other hand, technique for raising the color image quality is developed, by which a fine latent image is formed on the photoreceptor by using a light source with a small spot diameter to form a fine dot image. For example, a method is known in which a fine latent image is formed on an organic photoreceptor using a light beam having a spot diameter of not more than 4,000 μm2 such as that disclosed in Japanese Patent Publication Open to Public Inspection, hereinafter referred to as JP O.P.I. Publication, No. 8-272197.
It is necessary for exactly reproducing such the fine dot image as a toner image to prevent the lowering of the sharpness caused by lacking of toner transfer or scattering of character image caused by imperfect transfer and the periodical image defects caused by toner filming on the intermediate member.
For improving the charging property, developing ability and transferring ability, which are cause of the toner transfer lacking and the character image scattering, and for preventing toner filming or improving the imperfection of cleaning, techniques have been investigated by which fine particles are added into the photoreceptor layer to give irregularity to the surface thereof so that the toner adhesive force of the photoreceptor surface is reduced for improving the transfer ability and for reducing the frictional force of the surface to a blade. For example, JP O.P.I. Publication No. 5-181291 discloses that fine particles of alkylsilsesquioxane resin are added in the photosensitive layer. A problem rises, however, that the transfer ability tends to be lowered under a high humidity condition since the fine particles of alkylsilsesquioxane resin has hygroscopicity and the wettability of the photoreceptor surface or the surface energy of the surface is raised under such the condition. On the other hand, an electrophotographic photoreceptor containing particles of fluororesin for reducing the surface energy has been disclosed. However, sufficient surface strength cannot be obtained by the fluororesin particles and line-shaped defects caused by damage of the photoreceptor surface and image scattering tend to occur, cf. JP O.P.I. Publication No. 63-56658.
Besides, a technique for improving the transferring ability of the intermediate transfer member by supplying a solid lubricant to the intermediate transfer member to lower the surface energy is disclosed in, for example, JP O.P.I. Publication Nos. 6-337598, 6-332324 and 7-271142. It is found, however, that the solely controlling of the surface property of the intermediate transfer member is insufficient for improving the total transferring ability in the image forming method having the twice transfer processes using the intermediate transfer member, and further improvement is necessary regarding the copy image formation for a long period or under a high temperature and humidity condition.
From the viewpoint of the electrophotographic process, the image formation process is roughly classified into an analogical image formation using a halogen lamp as the light source and a digital image formation using a LED or laser as the light source. Recently, the main stream of the technology is rapidly changed, in the field of not only the printer for personal computer but the ordinary copy machine, to digital image forming method since the processing of image and the expansion to a complex image forming machine are easy realized.
Higher quality of the image tends to be required to the digital image forming method since such the method is applied for not only copying but formation of an original image.
Corresponding to such the requirement of the high quality image, it has been investigated that the latent image is conscientiously developed the latent image formed on the electrophotographic photoreceptor by using a small diameter toner controlled in the shape factor and the particle distribution. However, the transferring ability and the cleaning ability cannot be improved as expected and the lacking of toner transfer and the character image scattering tend to occur in the forgoing image forming method using the intermediate transfer member even when such the toner is used.
It is found that it is necessary to improve the toner transferring ability of both of the primary transfer and the secondary transfer in total by controlling the balance between the surface energy of the electrophotographic photoreceptor and that of the intermediate transferring member and improving the properties of the toner to suit to the intermediate transfer method.
Besides, an image forming apparatus is known as an embodiment of tandem method image forming apparatus for forming a color image, in which toner images are formed by each of electrophotographic photoreceptors each corresponding to color of yellow, magenta, cyan, and black, respectively, and the toner images are piled up on the recording medium to form a color image, cf. JP O.P.I. Publication No. 2001-222129.
In such the color image formation by the tandem method, image defects caused by imperfection of the toner transfer frequently occur since the toner images are piled up by directly transfer from the electrophotographic photoreceptor of each of the image forming units to the recording material or recording paper.
For example, the imperfection of the transfer tends to occur according to the kind of the recording paper, and the image defects such as the lowering of the image density, the lacking of toner transfer, and the scattering of character image caused by rebounding of the toner transfer occur, as a result of that the color image with lowered sharpness tends to be obtained. Particularly, the image quality is considerably degraded when a black image exists in the color image and the character image scattering caused by the rebounding in the course of transfer occurs in the blacken image.
In the color image forming apparatus, the ratio of formation of character images is larger than that of color images and the wearing of the photoreceptor and the degradation of the developer is easily progressed in the black image forming system. Therefore, it is important for the color image formation to maintain the charging ability, the developing ability and the transfer ability of the black image at a high level. Accordingly, it is necessary for leveling up the quality of the color image to mainly prevent the lowering of the sharpness and degradation of the color reproducibility caused by the toner transfer lacking and the character scattering accompanied with the degradation of the charging ability, the developing ability, and the transferring ability in the black image forming system.
As above-mentioned, in the color image forming system by the tandem method in which the toner image is directly transferred from the electrophotographic photoreceptor to the recording material, the foregoing improvement of the photoreceptor is insufficent to prevent the lowering of the image density and the image defects such as the lacking of toner transfer and the scattering of character image, and improvements of the developing ability and the transferring ability of each color toners, particularly those of the black toner, are necessary for improving the development of the electrostatic latent image on the photoreceptor and the transfer of the toner image from the photoreceptor to the recording material so that the toner image is sufficiently transferred to the recording material.
The invention is attained to solve the foregoing problems. The object of the invention is to provide a good electrophotographic color image by the image forming apparatus using the intermediate transfer member, particularly to provide an electrophotographic image forming apparatus and an image forming method by which the lacking of toner transfer the scattering of character image and the degradation of sharpness are improved, which are easy to occur in the color image formed by the apparatus using the intermediate transferring member, so as to form a color image with high sharpness and clear hue when the fine dot image or a lot of the images are formed.
The invention has been attained based on the detailed investigation on the primary transferring ability of the toner image from the photoreceptor to the intermediate transferring member and the secondary transferring ability of the toner image from the intermediate transferring member to the recording material in the image forming apparatus for forming a color image using the intermediate transferring member. As a result of the investigation, it has been found that the amount of the free external additive in each of the color toners largely participates with the primary and secondary transferring ability, particularly with the secondary transferring ability from the intermediate transferring member to the recording material. The transferring ability of the toner from the intermediate transferring member to the recording material is considerably improved by the use of a toner having a large amount of free external additive or high turbidity so that an electrophotographic color image having few image defects such as the transfer lacking and the character scattering and the high sharpness can be obtained.
The further object of the invention is to provide an image forming apparatus having plural image forming units and an image forming method by which the toner images each formed on the image forming units are directly transferred and piled up onto the recording material to form an electrophotographic image with good color, particularly, to provide the image forming apparatus and the image forming method by which the reproducibility degradation of the black image in a color image including a character image and the image defects such as the character image scattering and the toner transfer lacking are prevented so that a color image having high sharpness and color reproducibility is formed.
The invention is attained by the found that in an image forming apparatus in which toner images having different color from each other are formed by the use of color toners each charged in each of plural image forming units, and the toner images are successively transferred and piled up onto the recording material from the electrophotographic photoreceptor and fixed to form a color image, the amounts and their balance of the free external additive in the each of toner images formed by each color of the toners on the photoreceptor are largely participates with the developing ability and the transferring ability, particularly on the occasion of the each color toner images are transferred onto the recording material for piling up the toners on the recording material. Namely, the reproducibility of the black toner image can be held and the transfer ability of the toner images of each colors is considerably improved by the use of the toners in which the amount of the free external additive is reduced in the black toner compared with the other color toners and the amount of that is increased in at least one of the toners other than the black toner. Thus the electrophotographic image can be formed which has good color and high sharpness and few image defects such as toner transfer lacking and the character image scattering tending to occur in the color image.